What you say is true enough at moderate to high speed, but at very low speed, my Prius (2010) makes no engine sound while operating off the battery and very little tire noise. Actually, the electric system does make a high-pitched whining sound outside that my girlfriend can hear readily, most of the time and I can hear occasionally. But her hearing's a bit unusual-- she's a trained musician with a high-frequency cutoff somewhere around 30KHz (as opposed to the usual 20KHz or lower). She has managed to sneak up on me in the car on a couple of occasions.
Tire noise is dependent on what surface the car is on. There is none on dry, solid pavement, a sort of hissing sound from water being flung off the tires and hitting the insides of the wheel wells while on wet pavement. and the usual crunching, etc. sounds on gravel.
So, in a parking lot, driving at sane parking lot speeds, on dry pavement, and with the gas engine not having kicked in yet, there's not a lot to hear (unless you're a bat, a dog, a dolphin, or my girlfriend) and I am mindful of this.
I am torn between thinking that some kind of noisemaker wouldn't be a bad idea and a general distrust of yet more regulations. I'd probably buy a gizmo that generates a burst of engine-ish noise when I press a button, rather than being on all the time. I'd use it instead of the horn in those situations. I think it would be more polite. Save the horn for when they obviously know I'm there and are just walking down the middle of the row "because".